Recently, the portable wireless terminal becomes downsized and thin, and some techniques have been disclosed in which a metal case is used for the portable wireless terminals to ensure those rigidity. From a viewpoint of improved designs and protection from damages, the portable wireless terminals have been incorporating those antennas into themselves more and more. When a whole part of a case of the wireless terminal is made of metal, an antenna incorporated into the case does not operate. Therefore, techniques have been disclosed in which a part of the case is made of metal.
For example, in Patent Document 1, a device having a case of a portable wireless terminal, a part of which is metal, is disclosed. As shown in FIG. 45, the case of a wireless terminal device disclosed in Patent Document 1 is composed of a printed substrate 70 having an antenna element 71, and two cases 72 and 73 covering the antenna element 71 and the printed substrate 70 respectively. The case 73 covering the printed substrate 70 is a metal case and the case 72 covering the antenna element 71 is a plastic case, which ensures operation of the antenna and rigidity of the case.
In Patent Document 2, a coaxial resonant slot antenna in which a slot antenna is mounted on a metal case, and its manufacturing method are disclosed. A wireless terminal device disclosed in Patent Document 2 includes, as shown in FIGS. 46A, 46B, and 46C, an elongate belt-like conductor 75 disposed in an internal space of a flat conductor case 74, and an elongate slot 76 formed on the upper surface of the conductor case 74 orthogonally to the belt-like conductor 75 when seen in a planar view.
A connection point 79 between the belt-like conductor 75 and one end of a high-frequency circuit 77 is arranged at a position corresponding to a quarter wavelength of a usable frequency from one end 78 of the belt-like conductor 75. The other end of the high-frequency circuit 77 is connected to the conductor case 74. The belt-like conductor 75 and the metal conductor case 74 compose a coaxial line. When a signal with the usable wavelength is supplied from the connection point 79 to the belt-like conductor 75, a quarter wavelength resonance, with which electric field intensity becomes maximum at the end 78 of the belt-like conductor 75, and with which the electric field intensity becomes minimum at the connection point 79, is induced. Then, an electromagnetic wave forming this resonance is radiated from the slot 76 toward outside.
As shown in FIG. 47A, a small basic wireless antenna disclosed in Patent Document 3 performs induction at a slot 80 provided on a hollow metal conductor case 82 by using a probe 81 which is an extended part from a core portion of a three-branched line 84 connected on the hollow metal conductor case 82 through a connector 83.
With an induction method shown in FIG. 47A, impedance mismatching is caused. Therefore, as shown in FIG. 47B, an impedance matching circuit 86 is provided in between an antenna 85 and a main feed line 87, and the impedance between antenna 85 and the main feed line is adjusted by the impedance matching circuit 86.
Patent Document 4 discloses a method in which a matching circuit makes an antenna be a dual resonant antenna so as to extend an operating band of the antenna.
A dual resonant antenna device disclosed in Patent Document 4 has, as shown in FIGS. 48A and 48B, an antenna element 88 and an LC parallel resonant circuit 95 to make the antenna element 88 resonant in a plurality of frequency bands. The LC parallel resonant circuit 95 includes an inductance element 90 as a shunt element to prevent the impedance from reaching an infinite value in a prescribed frequency band, and a T-shaped circuit composed of capacitance elements 93 and 94. Further, an inductance element 91 is connected in between a feeding point 97 and a ground to match an input impedance of the antenna element 88 and the impedance of a feed circuit 96.
When the antenna element 88 is powered from the feed circuit 96 through the feeding point 97, as for a frequency characteristic, a band-pass characteristic S21 of the dual resonant antenna device does not have drop points in gain at two resonant frequencies f1 and f2, as shown in FIG. 48B, and therefore gain degradation can be prevented. According to this conventional technique, the impedance can be matched in a plurality of frequencies when a matching circuit having the LC parallel resonant circuit as a basic component is added to an antenna having a single resonant characteristic.
Patent Document 5 discloses a notch antenna, which is a slot with one side of it open, as a technique to minimize a slot antenna. As shown in FIG. 49, Patent Document 5 discloses a slit in a length corresponding to the quarter wavelength of a usable frequency provided on a substrate, which operates as an antenna. That is, as shown in FIG. 49, a notch antenna 104 is a linear slit in an electrical length corresponding to the quarter wavelength of the usable frequency, provided from an edge 103a of a substrate 103. The notch antenna 104 is provided with a feed section 105 for induction. Further, a notch antenna 106 is provided on the substrate 103, an interval between it and the notch antenna 104 is a distance d. The notch antenna 106 operates by electromagnetic coupling with the notch antenna 104, and is formed as a linear slit slightly shorter than the quarter wavelength of the usable frequency.    Patent Document 1: Japanese Patent Application Laid-open No. 2000-269849    Patent Document 2: Japanese Patent Application Laid-open No. 09-74312    Patent Document 3: Japanese Patent Application Laid-open No. 05-199031    Patent Document 4: Japanese Patent Application Laid-open No. 2003-249811    Patent Document 5: Japanese Patent Application Laid-open No. 2004-56421